The outer membrane of Escherichia coli serves as the initial permeability barrier of the cell, allowing passive diffusion of hydrophylic molecules. It is composed primarily of proteins, phospholipids and lipopolysaccharides. We have isolated a variety of mutant strains whose outer membranes are both missing major proteins normally present and contain new major proteins not previously described. We have examined the phenotypic effects on the cell of the loss of major outer membrane proteins and how these effects have been compensated for by the appearance of the new major outer membrane protein that we call protein E. Protein E restores the selective permeability of the outer membrane to small hydrophylic molecules lost in mutant strains missing certain major outer membrane proteins. We have purified and characterized protein E, isolated new bacteriophages which specifically use protein E as a receptor, shown that protein E is a bacteriophage-receptor for other bacteriophages, and prepared purified antibodies to both protein E and the new bacteriophages. We have also located a dominant gene, that we call nmpA, which is responsible for the phenotypic expression of protein E.